A side-effect is that their breath will smell sweet - a by-product of the body burning fat rather than sugar.

The pounding of feet and strain on muscles will hurt. Mike Stroud has started to "pass blood" in the urine. This will be the haemoglobin from red blood cells caused by the crushing of capillaries in the feet each time they hit the road. (Six out of the seven marathons are on hard surfaces.)

Small muscle tears will hurt and not have time to repair. One to two percent of muscle will be damaged after each race.

Another risk is stress fractures to the bones - tiny cracks in the feet and ankles. These would be "unbelievably painful" to run on.

Yet Sir Ranulph's years of arduous exploring will have built up strong bones, minimising the risk of such an injury, says physiologist Joe Dunbar.

The same goes for potential strains to tendons and inflamed knees, while their feet are probably tough enough to repel blisters. Cramp however, is inevitable - the combination of chronic overuse of muscles and the body's inability to absorb enough minerals.